[zendframework/zendframework#5569] Handle null values properly

- Previous fix led to:
  ```php
  'null' => ,
  ```
- Fix properly echoes the null value:
  ```php
  'null' => null,
  ```

src/Writer/PhpArray.php

@@ -82,6 +82,8 @@ protected function processIndented(array $config, array $arraySyntax, &$indentLe
                 $arrayString .= ($value ? 'true' : 'false') . ",\n";
             } elseif (is_string($value)) {
                 $arrayString .= "'" . addslashes($value) . "',\n";
+            } elseif ($value === null) {
+                $arrayString .= "null,\n";
             } else {
                 $arrayString .= $value . ",\n";
             }

test/Writer/PhpArrayTest.php

@@ -38,6 +38,7 @@ public function testRender()
             'object' => (object) array('foo' => 'bar'),
             'integer' => 123,
             'boolean' => false,
+            'null' => null,
         ));
 
         $configString = $this->writer->toString($config);
@@ -56,6 +57,7 @@ public function testRender()
         $expected .= ")),\n";
         $expected .= "    'integer' => 123,\n";
         $expected .= "    'boolean' => false,\n";
+        $expected .= "    'null' => null,\n";
         $expected .= ");\n";
 
         $this->assertEquals($expected, $configString);